Shanjin Huang

A chip-level electrothermal-coupled design model for high-power light-emitting diodes

An advanced three-dimensional electrothermal-coupled simulation model basing on finite-element method numerical simulation is developed to study the electrical and thermal properties of chip-level high-power GaN-based light-emitting diodes (LEDs). The current spreading, heat generation, and transfer in the device are comprehensively considered in this model. The current-spreading effect of the transparent current-spreading layer and the thermal performance of LEDs with interdigitated-electrodes are investigated. The simulation results prove that the temperature distribution in the active layer is strongly affected by the electrode pattern. The obvious heat accumulation in LEDs with conventional interdigitated-electrode patterns can be seen both in the simulated results and the infrared measured results. The heat transfer efficiency can be improved by using a symmetry electrode pattern design. The thermal management of the bump configurations in flip-chip LEDs is also studied. A more reasonable and thermal...

Enhanced electrostatic discharge properties of nitride-based light-emitting diodes with inserting Si-delta-doped layers

We report significantly improved electrostatic discharge (ESD) properties of InGaN/GaN-based light-emitting diodes (LEDs) with inserting Si-delta-doped layers between multiple quantum wells and n-cladding layer. The ESD endurance voltage increased from −1200 V to −4000 V with the insertion of delta-doped layers. The mechanism of the enhanced ESD properties was then investigated. According to capacitance-voltage results, the factor of capacitance modulation was ruled out. However, infrared microscopy image proved better current spreading in the LEDs with delta-doped layers. In addition, current-voltage, photoluminescence, and atomic force microscope measurements demonstrated substantial quality improvements. These two reasons were considered as the dominant mechanisms of the enhanced ESD properties.

Lateral Current Spreading Effect on the Efficiency Droop in GaN Based Light-Emitting Diodes

The lateral current spreading (CS) effect on the efficiency droop in GaN-based LEDs has been studied in terms of the CS distance (LCS) using a designed pattern with the 2-D current spreading profile. The correlations of CS effect with the electrical, luminescent and electric-thermal properties of the LEDs have been discussed. LEDs with the LCS longer than the theoretically calculated effective CS length (Leff) suffer from more serious efficiency droop and the degradation of luminescent properties. However, the influence of CS effect on the ideality factors of LEDs is not obvious. Higher chip temperature caused by poor CS was observed and may further enlarge the efficiency droop.

Contrary luminescence behaviors of InGaN/GaN light emitting diodes caused by carrier tunneling leakage

The luminescence properties of InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) with different quantum-well (QW) thicknesses were investigated. It is found that with decreasing the QW thicknesses, the integrated intensities of the photoluminescence (PL) and electroluminescence (EL) peaks demonstrate a contrary changing trend. The PL results show that the luminescence efficiency is improved by using thinner QWs. However, in the EL process, such a positive effect is counteracted by the low carrier injection efficiency in the thin QW LEDs, and consequently leads to a lower light output. Based on our experimental results, it is inferred that the tunneling leakage current associated with dislocations should be responsible for the low carrier-injection efficiency and the observed weaker EL integrated intensity of the LEDs with thin QWs.

Mechanical Properties of Nanoporous GaN and Its Application for Separation and Transfer of GaN Thin Films

Nanoporous (NP) gallium nitride (GaN) as a new class of GaN material has many interesting properties that the conventional GaN material does not have. In this paper, we focus on the mechanical properties of NP GaN, and the detailed physical mechanism of porous GaN in the application of liftoff. A decrease in elastic modulus and hardness was identified in NP GaN compared to the conventional GaN film. The promising application of NP GaN as release layers in the mechanical liftoff of GaN thin films and devices was systematically studied. A phase diagram was generated to correlate the initial NP GaN profiles with the as-overgrown morphologies of the NP structures. The fracture toughness of the NP GaN release layer was studied in terms of the voided-space-ratio. It is shown that the transformed morphologies and fracture toughness of the NP GaN layer after overgrowth strongly depends on the initial porosity of NP GaN templates. The mechanical separation and transfer of a GaN film over a 2 in. wafer was demonstrated, which proves that this technique is useful in practical applications.

Emission enhancement in GaN-based light emitting diodes with shallow triangular quantum wells

GaN-based light emitting diodes (LEDs) with shallow triangular quantum wells (TQW) structure were proposed and investigated. LEDs with shallow TQW demonstrated a lower turn-on voltage and 80% higher lighting efficiency at 20 mA than devices without the shallow QW structure. A more stable emission wavelength and a lower ideality factor were achieved with the proposed structure. X-ray reciprocal space mapping revealed a partial strain relaxation in active region due to the insertion of the TQW structure. The improved performance is ascribed to the weakening of the polarization field in the MQW active region induced by the TQW structure.

Improved carrier injection and efficiency droop in InGaN/GaN light-emitting diodes with step-stage multiple-quantum-well structure and hole-blocking barriers

Hole transport control and carrier injection improvement have been demonstrated in the InGaN/GaN light-emitting diodes (LEDs) with step-stage multiple-quantum-well (MQW) structure and Si-doped hole-blocking barriers. Single-wavelength emission was obtained under electrical pumping in these LEDs by utilizing hole-blocking effect. The light emission around 450 nm showed a substantial increase compared with the reference sample with single or step-stage indium-content MQWs. The droop behavior and wavelength stability were also improved significantly. These improvements were attributed to the enhanced carrier injection to the active region due to the alleviation of the quantum-confined Stark effect and the effective hole-blocking effect of the Si-doped barriers.

Analysis and modeling of the experimentally observed anomalous mobility properties of periodically Si-delta-doped GaN layers

We report the anomalous mobility properties of Si-delta-doped GaN with periodically doping profile. Samples with different delta-doping periods or with varied Si source flow were investigated. It is found that for the short delta-doping-period (<26.5 nm) samples, the Hall mobility increases with decreasing electron concentration; while for the longer-doping-period samples, the situation is just the opposite. To interpret this observation, a two-layer model has been built up for long-period samples based on secondary ion mass spectroscopy measurements. The fitting results using this model are well consistent with the experimental data.

GaN-Based LEDs With Al-Doped ZnO Transparent Conductive Layer Grown by Metal Organic Chemical Vapor Deposition: Ultralow Forward Voltage and Highly Uniformity

In this letter, InGaN/GaN multiquantum well LEDs were fabricated with aluminum-doped zinc oxide (AZO) transparent conductive layer (TCL) grown by metal organic chemical vapor deposition (MOCVD) on n+-InGaN contact layer. Ultralow forward voltage (Vf) of 2.86 V at 20 mA was obtained, attributing to the epitaxial-like excellent interface between AZO/n+-InGaN contact layer confirmed by high-resolution transmission electron microscopy. The most worthy is that the Vf uniformity can be demonstrated on a 2-in wafer with a standard deviation of 0.02 V. Combined with the excellent light extraction, the MOCVD grown AZO-TCL is expected to be an alternative of tin-doped indium oxide in GaN-based LEDs for mass production.

Role of InGaN Insertion Layer on Nitride-Based Light-Emitting Diodes

In this study, we systematically investigate the effect of InGaN insertion layer (IL) on nitride-based light-emitting diodes. First, a series of samples with different InGaN ILs (Si doping level, thickness) were fabricated and investigated. An optimized condition of the IL was obtained based on current–voltage and electroluminescence measurements. Furthermore, in order to investigate the dominant mechanism for the improved performance of the samples with IL, the optimized sample and a control sample without InGaN IL were compared by means of X-ray diffraction, atomic force microscopy, photoluminescence, injection-current-dependent electroluminescence measurements and infrared camera images. Based on the discussion of these measurement results, we conclude that the performance improvements of samples with InGaN IL are due to both the effects of strain relaxation and better current spreading.